What happened to all of the oil?

Oil from the Deepwater Horizon on the surface of the Gulf of Mexico. (c) ECOGIG

October 06, 2016

This paper, written by Dr. Uta Passow (ECOGIG) and Dr. Robert Hetland and published in the GoMRI special issue of Oceanography, summarizes a large body of research from a number of papers in the attempt to answer the question "what happened to all of the oil spilled during the Deepwater Horizon accident?" This accident and subsequent oil spill was unique in that it originated at a great depth and persisted for an extended period of time, which released a large quantity of oil (approximately 5 million barrels) and natural gas (approximately 7.7 billion cubic feet) into the environment.

The problem with this seemingly simple question is that the Gulf of Mexico is very large and the water is constantly in motion, so finding anything is very difficult. This problem is compounded by the fact that water at different depths moves in different directions, carrying things with it as it moves, such as oil. To make the situation even more complex is the fact that oil is made up of hundreds of different compounds, each of which behaves differently and the behavior depends on things like ocean temperature and pressure (water depth). Some compounds evaporate easily, others float and others remain at certain depths. Some of these substances are easily consumed or altered by bacteria, some are changed by sunlight and others remain the same for decades. It is impossible to follow oil itself over time- we can only follow a specific compound of oil. However, because every kind of oil is dfferent, it is often difficult to know how oil compounds change. The additional of dispersants that affect oil distribution makes the situation even more complex.

On top of all of that, biological processes also impact oil distribution in the water. When oil is swallowed by organisms or coats them, it moves with the organism. When oil is trapped within sinking marine snow formed by microbes, oil sinks to the bottom. This amazing complexity means that it is impossible to really know what happened to all the oil. The fact that we do not even know exactly how much oil was spilled makes a real oil budget impossible. This article attempts to do some crude estimates on the relative importances of the different fates of oil, taking a birds eye view of what happened to all of the oil.